Ki is the energy permeating all living things and not having Ki is tantamount to being dead. "Ki" is the Japanese pronuciation of the word, the dominant if idiosyncratic way to represent the Mandarin Chinese pronunciation (pinyin qì) in English writing is Chi (see the wikipedia article).
Ki exists in the types of Yin and Yang Ki and both halves should be in balance. Extreme imbalances can result in death, which is likely the reason natural Ki mutants are virtually unknown, Chaka being the exception and having the fortune to have a safety valve of sorts, spasming out in a "fit" and bleeding off the excess Yin (usually) rather than dying.
Control of Ki is central to many eastern martial arts and awakening ones Ki and being able to use it consciously the basis of many higher level techniques, but reaching that level usually takes many years or even decades. Arts particularly focused on and helpful in learning the control of Ki include Aikidō and Qi Gong (which both include the word in their name) as well as Tai Chi Ch'uan (which doesn't).
Ki appears to be closely related to essence and like the latter can flow along ley lines, sometimes referred to as dragon lines in this context. Some martial arts techniques involve pushing Ki into these lines.